1. Field of the Invention
This invention relates to a power unit which generates a single-phase AC power having a commercial frequency or a like frequency.
2. Prior Art
Conventionally, a power unit which is a combination of a small-sized engine and a synchronous generator, for instance, is widely used for emergency purposes, outdoor works, leisure time amusement, etc.
In this type of conventional power unit, however, the output frequency depends on the rotational speed of the engine. Therefore, in the case of a bipolar generator, to obtain an AC output of 50 Hz (or 60 Hz), the rotational speed of the engine is required to be held at 3000 rpm (or 3600 rpm), i.e. a relatively low rotational speed, which degrades the operating efficiency of the power unit, and further, necessitates designing the generator to be large in size, resulting in an increased total weight of the power unit.
To overcome this inconvenience, a so-called inverter generator has been proposed by the present assignee, e.g. in Japanese Patent Publication (Kokoku) No. 7-67229 and Japanese Laid-Open Patent Publication (Kokai) No. 4-355672, which is constructed such that the engine is operated at a relatively high rotational speed to obtain a high AC power from the generator, the AC power is once converted to direct current, and then the direct current is converted to alternating current having a commercial frequency by an inverter.
The conventional inverter generator, however, requires provision of two power conversion blocks, i.e. an AC-to-DC conversion block for once converting the AC power to DC power, and a DC-to-AC conversion block for converting the DC power to AC power having a predetermined frequency, as well as a circuit for temporarily storing the DC power. Thus, the use of a lot of expensive power circuit components is necessitated. This makes it difficult to reduce the size of the generator and leads to an increased manufacturing cost.
As a solution to this problem, a so-called cycloconverter generator has become commercially available, in which a cycloconverter is employed for use with the generator to directly convert the high AC power generated by the generator (the generator is operated at a relatively high engine rotational speed, and hence the frequency of the alternating current generating the AC power is higher than a commercial frequency) to AC power having the predetermined commercial frequency, without carrying out AC-to-DC conversion.
In the conventional cycloconverter generator, however, since the AC power generated by the generator is directly converted to the AC power having the predetermined frequency (commercial frequency), without being once converted to direct current, as described above, to promptly reduce an undue fluctuation in the output voltage caused by a rapid change in the input voltage which inevitably occurs when a generator having a relatively small capacity is used, or more specifically, an undue fluctuation in the output voltage occurring when the power unit is switched between a no-loaded condition thereof and a loaded condition thereof, in short, to reduce an output voltage regulation, a very large feedback gain is required of the power unit.
Therefore, when a conventional control method, or more specifically, a control method of reducing the output voltage regulation simply by feedback of the waveform of the output voltage is applied to the above conventional cycloconverter generator, a very large feedback gain is required, as mentioned above, which makes it difficult to achieve stable control of the generator.
A possible solution to this problem is to modify the cycloconverter generator such that the effective value of the output voltage is detected over a predetermined number of repetition periods of the output voltage and feedback control is carried out based on the detected effective voltage value, thereby reducing the feedback gain to enable the generator to perform more stable feedback control.
The cycloconverter generator modified as above can sufficiently cope with a fluctuation in the output voltage caused by a rapid change in the input voltage occurring when the power unit is switched between a no-loaded condition thereof and a loaded condition thereof. However, the AC power is generated from a generator driven by an engine operating with a relatively high rotational speed which is changing, and such AC power is directly converted to AC power having a predetermined frequency (commercial frequency), without being converted to direct current. Therefore, it is impossible for the generator to sufficiently cope with fluctuations in the output voltage occurring according to fluctuations in the input voltage ascribable to fluctuations in the engine rotational speed. This inconvenience cannot be eliminated even if the effective value of the output voltage is detected by limiting the number of repetition periods of the output voltage to one, because a time period taken for detection of an effective value of the output voltage per repetition period thereof is considerably longer than a time period over which a change in the input voltage takes place according to a change in the engine rotational speed. More specifically, assuming that a single-cylinder four-cycle engine is rotated at 3600 rpm to drive the generator, and a rated load is connected to the power unit, the engine rotational speed varies to an extent of approximately xc2x1150 rpm from 3600 rpm during an explosion stroke of the engine. This variation (fluctuation) takes place over a time period of approximately 5 msec. On the other hand, assuming that the frequency of the AC output of the cycloconverter generator is a commercial frequency, i.e. 50 Hz, a time period taken for the detection of an effective value of the output voltage per repetition period thereof is 20 msec. This means that when a factor has been detected to carry out effective feedback control based thereon, a change in the engine rotational speed responsible for the factor, which is to be controlled, has already ended, and hence even the method based on detection of the effective value of the output voltage per repetition period cannot enable the power unit to sufficiently cope with fluctuations in the output voltage occurring according to fluctuations in the input voltage caused by fluctuations in the engine rotational speed.
It is an object of the present invention to provide a power unit which is capable of performing stable feedback control by reducing a feedback gain, and at the same time, sufficiently coping with fluctuations in the output voltage occurring according to fluctuations in the input voltage caused by fluctuations in the rotational speed of an engine as a drive source of the power unit.
To attain the above object, the present invention provides a power unit comprising:
a three-phase generator having a three-phase output windings;
a pair of variable control bridge circuits connected to the three-phase output windings of the three-phase generator and connected in an antiparallel manner to each other to form a cycloconverter for generating a single-phase alternating current to be supplied to a load;
voltage-detecting means for detecting a voltage of the single-phase alternating current generated by the pair of variable control bridge circuits;
effective voltage value-detecting means for detecting an effective value of the voltage of the single-phase alternating current;
reference waveform voltage-generating means for generating a voltage having a reference waveform for controlling the single-phase alternating current;
reference effective voltage-generating means for generating a reference effective voltage for controlling the single-phase alternating current; and
control means for controlling the pair of variable control bridge circuits such that the pair of variable control bridge circuits alternately switch to operate every half a repetition period of the single-phase alternating current, so as to make the effective value of the voltage of the single-phase alternating current detected by the effective voltage value-detecting means closer to a value of the reference effective voltage generated by the reference effective voltage-generating means, and at the same time, make the voltage of the single-phase alternating current detected by the voltage-detecting means closer to the voltage having the reference waveform, which is generated by the reference waveform voltage-generating means.
Preferably, the effective voltage value-detecting means detects the effective value over a predetermined number of repetition periods of the voltage of the single-phase alternating current.
More preferably, the predetermined number of repetition periods of the voltage of the single-phase alternating current is one repetition period.
Preferably, the three-phase generator is a magneto generator having a permanent magnet rotor.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.